Dahlia, sounds like you’re on the right track. There are several ways to generate heat, electric being the least practical due to its huge power requirements. I use a solid fuel heater, similar to a wood burning stove, but it burns charcoal and coal (and can also burn wood in a pinch). It produces a nice dry heat without using any electricity. Most others use either a propane catalytic heater (doesn’t use electricity, but not a dry heat), or some kind of furnace-type truck heater such as a Webasto diesel heater or Propex propane heater. The advantage of the furnace-type heaters is that most of them have a thermostat that can be set, and produce dry heat, but the disadvantage is that they use electricity to run a blower (noisy) and the control circuitry, so if your power goes out the heater doesn’t work.
For cooling there are a few options: refrigerated air (air conditioning) uses a lot of power, so you’d need a generator or shore power. If you live in a dry climate evaporative coolers work well (swamp coolers), and don’t use a lot of electricity. They don’t work well in hot/humid areas because the cooling mechanism is dependent on evaporation of water - if it’s too humid the evaporation rate is too low to be effective.
Passive methods of cooling can also be effective, such as parking in the shade and providing plenty of ventilation through the van. If the outside temperature is comfortable, there are ways to keep the air inside the van about the same temperature.